Diamonds Crescent-Shaped Circles Triangles 
Answer: Circles
Back in the late 1940s, Joe Woodland invented what would become the modern bar code, but his invention took a curious path before it became the rectangle we recognize today. He came up with the idea while sitting on Miami Beach pondering how he could create a machine readable inventory label for grocery goods—a problem he was so focused on solving that he’d dropped out of graduate school to do so. He described the moment he came up with the bar code as such:
I remember I was thinking about dots and dashes when I poked my four fingers into the sand and, for whatever reason—I didn’t know—I pulled my hand toward me and I had four lines. I said ‘Golly! Now I have four lines and they could be wide lines and narrow lines, instead of dots and dashes. Now I have a better chance of finding the doggone thing.’ Then, only seconds later, I took my four fingers—they were still in the sand—and I swept them round into a circle.
Although the first design he drew in the sand was the rectangular and linear design we’re familiar with today, the first prototypes and tests of the bar code were just as Woodland envisioned it: circular. Those early tests didn’t go anywhere, however, because Woodland’s design was so far ahead of existing technology that it couldn’t yet be fully utilized—it would be decades before the advent of cheap lasers and cheap microcomputers would make automatic checkout scanning feasible.
In the mid 1960s, however, when the National Association of Food Chains (NAFC, which later became the U.S. Supermarket Ad Hoc Committee on a Uniform Grocery Product Code) began searching for a way to automate grocery store checkouts, RCA (who owned the rights to Woodland’s design) brought his bullseye design out of storage and fine tuned it for testing in the Kroger grocery chain. While the system worked, a significant problem was quickly discovered. Any smearing of the label ink, damage to the label, or imperfections rendered the circular design unreadable by the system. Further tests, conducted by RCA, IBM, and other companies competing for the NAFC bid, revealed that the rectangular barcode was superior in terms of resistance to smearing and damage as even imperfect rectangular barcodes could still usually be read by the scanners.